It is well known in the art that humidifiers are used with respiratory equipment to both warm and humidify the breathing gas provided to a patient. The breathing gas can be oxygen, air, anesthetic gas or mixtures thereof.
Frequently, the breathing gas is composed of a mixture of air and augmenting oxygen. It has been found to be much simpler to have the breathing apparatus control the volume of only one of the gases and have the correct ratio of the other gas provided by aspiration through a venturi aspirator. In order for this mixing technique to operate correctly, the back pressure in the system must be minimized. Many humidifiers provide excessive back pressure so that their use in conjunction with a venturi-type oxygen mixing aspirator is impractical.
Another area of concern in humidifier design is to provide equipment which can be sterilized and/or incorporate elements which are replaced after usage to prevent transfer of contagion between patients. Conditions within a respiratory circuit are conducive to very rapid growth of pathogenic microorganisms which can be carried into the circuit during the patient's exhalation cycle and thrive and multiply within the warm, moist atmosphere. If the assembly is not thoroughly sterilized before reuse, contagion can be easily blown directly into the next patient who may be in weakened condition and thus ill equipped to cope with same.
Still another area of concern in the design of respiratory humidifiers is to provide a unit which will provide adequate heating and humidification for a wide range of gas flows from the very low flow required for infants to the highest flow required by adults. The humidifier unit should also be capable of supplying almost instant heating and humidification so that it is ready for instant use in an emergency without the need for a warm-up period and so that steady state can be reached quickly minimizing temperature variations resulting from an extended warm-up period which can vary from 10 minutes to as along as 60 minutes in presently available humidifiers.
Respiratory systems frequently incorporate an artificial breathing apparatus which generates pulses of breathing gas to the patient through flexible connective tubing. The breathing apparatus can provide either pulses of a given pressure or of a given volume. The apparatus can be set to provide pulses which are adjusted to fit the particular patient, whether juvenile with small lung capacity or adult with large lung capacity. Normally the breathing apparatus is located some distance away from the patient so that the interconnecting tubing comprises a considerable volume which makes accurate control of the pulses difficult. The humidifier is normally located intermediate along the interconnecting delivery tubing so that the internal volume of the humidifier also contributes more or less significantly to the interconnection volume. Accordingly, it would be advantageous to make the internal volume of the humidifier as small as possible.
When the humidifier is used in conjunction with an artificial breathing apparatus it will obviously have pulses of breathing gas passing through it. These pulses of gas have an instantaneous flow rate which starts at zero at the beginning of the pulse, increases to a maximum value, and decreases again to zero at the end of the pulse. This maximum flow rate is termed the "peak inspiratory flow rate."
A humidifier can typically accommodate peak inspiratory flow rates that exceed its maximum continuous flow rate since the humidifier pre-heats the volume of gas residing within the humidification chamber between pulses in addition to heating the pulsed gas as it passes through the humidifier. The "dead" or resident gas will thus be heated to a higher temperature than the "pulses" gas and will be mixed in the humidification chamber, the mixed humidified gases at the humidifier outlet being generally of uniform temperature and pulsatile.
Medical humidifiers generally fall into three types or classifications: (1) Nebulizers or droplet spray types, (2) Bubbler types, and (3) Evaporative or steam types. Other types or variations have been proposed from time to time but the three categories which will be discussed are those which are most widely used and accepted at the present time.